The present invention relates to the field of boat hull design, and, more particularly, to efficiency and speeds improvement upon V hulls without substantially altering well known and widely accepted V hull handling characteristics and aesthetics.
It should be noted that, as used herein, V hull refers not only to the conventional V hull, but also to all modifications of it that employ the same general hull configuration, particularly below the waterline. Therefore the so-called "cathedral" hull is included, since in dynamic, steady state, straight line operation it acts as a V hull and the data hereinafter recited applies generally to it.
The V hull the is most commonly used hull design, which may be modified for particular applications, as disclosed by Wollard, U.S. Pat. No. 3,469,557, Stuart, U.S. Pat. No. 3,515,087, and Cole, U.S. Pat. Nos. 3,602,179, D219,627 and D224,275. Such hull designs are popular in part because the design is relatively simple, the bow is effective in cutting through waves, as recited by Bremer, U.S. Pat. No. 3,930,455, and because the V hull offers a smooth ride, seaworthiness and good handling and steering characteristics as recited by Stuart and Wollard.
If the V is flattened out towards the stern, it may be to permit greater efficiency, since there is a lesser amount of wetted surface which reduces drag. Of course, the flatter the overall hull, the less the drag and the greater the speed, but the rougher the ride. Actually most hull design is a compromise between efficiency and comfort, and boats intended for rough water use often have a deep V in the hull notwithstanding the greater drag that results.
Drag on any boat hull is made up of several components. Induced drag is made up of drag due to lift and spray drag, and is a major component with a conventional V hull. Frictional or wetted area drag is a second major component. Appendage drag results from rudders, propellers, struts, etc. Aerodynamic drag is generally a very small component.
Efforts to improve efficiency of boat hulls naturally focuses on ways to reduce drag in any form.
One well known way to improve efficiency has been use of the hydrofoil craft. Hydrofoils achieve greatly increased speed for equivalent power consumption and, to some degree, avoid roughness resulting from waves, but suffer from fouling, strains resulting from the cantilevered posture of the foils, deep draft at rest, high maintenance, docking difficulties, and are highly inefficient at less than foil speed.
A major effect to reduce drag was the introduction in England several decades ago of a water craft intended to hover over the surface on a cushion of air. The air leaked beneath a skirt that surrounded the base of the craft. One significant shortcoming of this device was that power consumption to replace the air that leaked from the entire 360 degree skirt was too great to make the device practical and efficient, at least for many applications. Sideslipping and stability were also problems. However, hydrodynamic drag components could be largely eliminated.
Compromise solutions were also sought, often by introducing air between the hull and the water in some manner. So called air cushion vehicles were then developed, including a subcategory of the latter sometimes referred to as the captured air bubble craft, with which art the present invention deals.
Ford, U.S. Pat. No. 3,146,752, Beardsley, U.S. Pat. No. 3,140,687, Mantle et al, U.S. Pat. No. 3,476,069, and Crowley, U.S. Pat. No. 3,742,888 are all examples of this genus of boat hull invention. All suggest partial submergence of the sidewalls or skirts of the craft, but rely principally or entirely upon an air cushion for support of the craft when underway, and utilize the sidewall or skirt submergence largely as a seal to minimize escape of the supporting air cushion. Such devices are still primarily vehicles supported by air and represent significant departures from conventional and widely used V hull designs.
The extreme may well be Gunther, U.S. Pat. No. 3,473,503, which appears to be highly complex and expensive, consuming virtually all usable hull space, and with gross aesthetic effects, notwithstanding asserted amphibious capability.
Moreover, all of the foregoing air cushion inventions, including Ford, Beardsley, Mantle et al, Crowley and Gunther, do not appear to be stable in other than smooth water in small sizes, draw a great deal of water when at rest, and are not practical in small sizes appropriate to pleasure craft. In fact, the U.S. Navy is known to prefer the largest sizes of this type of craft, called by them the "Surface Effect Ship", or SES, and is known to have established a research program for development of large SES's, as disclosed in the May 31, 1973 issue of Machine Design, pp. 20-25. Also, many of the air cushion designs include expensive, high maintenance seals at the waterline, and some, such as Ford, use complex, movable seals.
There have also been efforts to adapt airfoils to other types of hulls, such as Walker, U.S. Pat. No. 3,477,400, with a hydrofoil, and Szptyman, U.S. Pat. No. 3,702,598, with a customized tunnel hull. In Walker, any significant aerodynamic effect from the airfoil surface would have to arise at substantial forward speed relative to the ambient air since there is no other technique disclosed for air movement or pressurization. By comparison, Szptyman optionally offers a blower to force feed air to the aerodynamic lifting surface, but the tunnel is still open to the atmosphere, thus requiring substantial forward speed relative to the ambient air much as with Walker. Neither reference is a "captured air bubble" design.
Of course, both Walker and Szptyman also represent significant departures from conventional and widely used V hull designs, and Szptyman consumes much usable hull space.
Glass, U.S. Pat. No. 3,547,064 is a modified form of this class of invention, except that it receives no support from whatever air cushion produced. While the hull exterior in Glass provides a semblance of an air chamber, that air chamber is open to the atmosphere in a manner similar to Szptyman, thereby providing no lift, reduction of drag resulting from less induced drag, etc.
The closest known art is Von Heidenstam, U.S. Pat. No. 3,331,347, which includes the concept of a blower connecting through duct work to a recess in the aft portion of a hull. Von Heidenstam, however, provides for an air chamber aimed at reducing friction drag through the interpositioning of an air layer betewen a substantial portion of the wetted hull area and the water's surface. Von Heidenstam is incapable of preventing lateral leakage of the air from the sides of the air chamber because it lacks structure to do so, and is not truly a "captured air bubble" concept because there is no capturing of the air. Von Heidenstam lacks the ability to significantly reduce induced drag that results from lift because there is no sharp or substantially right angle intersection of the air chamber with the exterior of the hull surface, there are no extended side walls, the side walls are not parallel to the keel, and the side walls are specifically recited in Von Heidenstam to terminate at or near the "bilge lines" or chines which is extremely close or at the water line of the boat.